The present invention relates to Stirling cycle machines and, in particular, it concerns Stirling cycle system having displacer driven by a synchronous linear motor with a magnetic spring.
It is known to employ Stirling cycle systems for cooling. Stirling cycle systems contain two essential moving parts, both of which execute reciprocating motion. One of these parts, usually known as the "displacer", is typically implemented as a plunger movable with clearance along a cylinder. This serves to transfer a mass of gas in alternate directions between the two ends of the cylinder. The displacer is connected to a compressor which generates pulsating pressure within the gas. By driving the displacer at the same frequency as the compressor, but at a certain phase difference therefrom, the system operates as a heat pump with heat being removed from one end of the displacer. In this manner, Stirling cycle systems can be used as the basis for a range of refrigeration systems.
In order to maintain efficient operation of a Stirling cycle system, the phase relationship between the compressor and the displacer must be optimized. The phase relation is most commonly maintained by a mechanical linkage between the compressor and displacer. However, it is very difficult to make such a linkage so as to allow adjustment of either the phase relation or the amplitude. In addition, a mechanical linkage tends to transmit vibration between the two parts of the system, rendering both the linkage itself and the system as a whole unreliable.
An alternative approach is a split structure employing a "passive" displacer assembly in which the displacer acts as a free piston. By designing the displacer assembly to achieve the right natural frequencies of oscillation, the displacer responds to the compressor output with movements that show the right difference in phase from those of the compressor itself. Calibration and adjustment of passive displacer systems, however, are difficult to achieve. An example of such a system is U.S. Pat. No. 4,862,695 to Kushnir in which a complicated three-spring damping mechanism is provided to allow adjustment of the displacer movement.
In an attempt to achieve improved control without the disadvantages of a mechanical linkage, U.S. Pat. No. 4,397,155 to Davey describes a Stirling cycle machine with an "active" displacer assembly in which the relationship between the movements of the displacer and the compressor is controlled by an independent electromagnetic device, in one embodiment functioning as a motor. The motor includes a coil carried by the displacer plunger and movable within the field of a stationary magnet.
The Davey machine has a number of advantages. However, the efficiency of the system is severely limited by the motor structure employed. Specifically, the motor is shown as a single cylindrical coil moving axially within the cylindrical gap of a permanent magnet circuit. The motor structure described provides low efficiency and has no self-centering properties. Such a system also requires sliding contacts which are unreliable.
Furthermore, the displacer of the Davey machine, in common with all other Stirling displacers, requires various spring elements to return the displacer to its neutral position. Such mechanical spring elements present numerous design problems, being difficult to adjust, intensifying mechanical wear, and increasing acoustic noise.
There is therefore a need for a Stirling cycle system having a motor driven displacer which has a magnetic spring effect without the need for mechanical contact. It would also be advantageous to have a Stirling cycle system which includes such a displacer.